Macroporosity and Grain Density of Rubble Pile Asteroid (162173) Ryugu

The carbonaceous asteroid (162173) Ryugu formed from fragments which re-accreted after its parent body was disrupted by a catastrophic collision. Asteroids of this type are also known as rubble piles and the re-accumulation process is thought to be one of the causes for their large bulk porosity. We have applied mixing models to determine the amount of inter-boulder porosity taking the observed abundance of large and small boulders on the surface into account. We find that the relative abundances of differently sized boulders allow for a very efficient packing, such that inter-boulder porosity in Ryugu is rather small and only $16 \pm 3$~\%. This implies that a large part of { Ryugu's} total porosity must reside inside the boulders themselves. Using estimates of boulder intrinsic porosity, we furthermore constrain the average density of the boulder's constituent minerals to { $2848 \pm 152$ kg m$^{-3}$}, which is consistent with values measured for carbonaceous meteorites as collected on Earth. Thus, inter-boulder porosity of rubble pile asteroids may have been systematically overestimated in the past